1. Field of the Invention
The present invention relates to an apparatus configured to generate and detect a terahertz wave to measure a time-domain waveform of the terahertz wave and a tomographic image acquisition apparatus using such an apparatus. More particularly, the present invention relates to an apparatus configured to generate and detect a terahertz wave using a single device (hereinafter such an apparatus will be referred to as a transceiver) and a tomographic image acquisition apparatus using such a transceiver.
2. Description of the Related Art
A terahertz (THz) wave is an electromagnetic wave with a frequency in an arbitrary frequency band within a range from 0.03 THz to 30 THz. This frequency range includes frequencies or bands of frequency at which characteristic absorption occurs due to structures or states of substances such as biological molecules. This characteristic absorption feature is useful to nondestructively analyze or identify a substance, and thus associated techniques have been developed. One example of an expected application is a safety imaging technique that can enhance or potentially replace an X-ray imaging technique. Another example of an application of terahertz waves is the development of a high-speed communication technique.
When a terahertz wave used is in the form of a pulse, it is known to sample the terahertz wave using excitation light in the form of an ultra-short light pulse thereby measuring the terahertz wave reaching a detector. This technique is called THz-TDS (THz-Time Domain Spectroscopy). In many THz-TDS apparatuses, a photoconductive device is used as a generating device or a detecting device because of its high efficiency in generation and detection of terahertz waves. The photoconductive device may be produced using a semiconductor film (also referred to as a photoconductive film in the present description) on which electrodes including an antenna are formed. More specifically, the electrodes are disposed so as to oppose each other via a gap (also referred to as an excitation light illumination region in the present description). The excitation light illumination region is illuminated with an ultra-short light pulse to instantaneously make the gap between the two electrodes conductive thereby generating or measuring a terahertz wave by using the sampling technique. WO2001/077646A1 discloses a terahertz wave transceiver, which is an example of a THz-TDS apparatus, configured to generate and detect a terahertz wave using a single photoconductive device. In this terahertz wave transceiver disclosed in WO2001/077646A1, excitation light is split into to two beams of light, i.e., pump light and probe light and a terahertz wave is generated by the pump light. The intensity of the generated terahertz wave is modulated using a chopper, and a current signal generated by probe light illuminating the gap portion and the electric field of the terahertz wave is detected using a lock-in detection method.
In the configuration of the apparatus disclosed in WO2001/077646A1, the current signal is detected from the excitation light illumination region while applying a voltage to the excitation light illumination region of the photoconductive device thereby generating the terahertz wave. Therefore, the current detection unit, which detects the current generated in the excitation light illumination region, receives a current I generated by the voltage used to generate the terahertz wave in addition to the terahertz-wave current δi generated by the electric field of the terahertz wave. For example, in a case where low-temperature grown gallium arsenide (LT-GaAs) is used as the photoconductive film of the photoconductive device, the terahertz-wave current δi is as small as a few nA to a few ten nA, but the current I is a few μA to a few ten μA, which is greater than the terahertz-wave current δi. To detect these currents, the current detection unit may be configured to have a high sensitivity as possible within a range in which no saturation occurs in the output of the current detection unit. However, in the technique disclosed in WO2001/077646A1, the large value of the current I relative to the terahertz-wave current δi makes it difficult to detect the terahertz-wave current δi with high sensitivity.